1. Field of the Invention
The present invention relates to a duplexer fabricated with a film bulk acoustic resonator (FBAR), and a method for manufacturing the same. More particularly, the present invention relates to a duplexer fabricated with a monolithic FBAR and an isolation part, and a method for manufacturing the same.
2. Description of the Related Art
As the use of mobile communication devices, represented by a mobile phone, has been increasing recently, studies and efforts for higher performance of each component and miniaturization of the devices have increased.
One of the main component parts of the mobile communication devices is a duplexer which utilizes a plurality of filters. The duplexer splits signals for reception and transmission via a single antenna in a frequency division duplex (FDD) type communication system such that one antenna can effectively be shared.
Besides the antenna, the duplexer includes a transmitter filter and a receiver filter. The transmitter filter is a band pass filter which passes only a signal in a frequency band for transmission, and the receiver filter is a band pass filter which passes only a signal in a frequency band for reception. The duplexer can perform the signal transmission and reception on the same antenna by varying the frequencies passing through the transmitter filter and the receiver filter.
For the transmitter and receiver filters, which are the basic components of the duplexer, a film bulk acoustic resonator (FBAR) can be employed since it can be mass-produced at minimum cost and can be implemented in minimum size. In addition, the FBAR enables a high quality factor (Q) which is a special feature of the filter, and can be used in a micro frequency band. Especially, the FBAR is able to realize even a personal communication system (PCS) band and a digital cordless system (DCS) band.
The FBAR element generally comprises a lower electrode, a piezoelectric layer, and an upper electrode deposited in order, and resonates as an external electric field is applied. In other words, when an electric field that changes with time is induced in the piezoelectric layer from on electric energy applied to the upper and lower electrodes of the FBAR, the piezoelectric layer causes piezoelectricity where the electric energy is converted to an acoustic wave mechanic energy, thereby generating resonance. Since only signals in a predetermined band with respect to the resonance frequency can pass, the FBAR operates as a band pass filter.
For better performance of the duplexer which splits signals received and transmitted via one antenna, inter-signal interference should be prevented. Since a difference between the frequencies of the signals transmitted and received through the transmitter filter and the receiver filter is small, the signals are quite sensitive to the inter-signal interference. Accordingly, the duplexer can have an improved performance by adding an isolation part which can prevent the inter-signal interference by isolating the transmitter filter and the receiver filter from each other.
The isolation part implements a phase shifter using a capacitor and an inductor to prevent the inter-signal interference by making the phase difference between the frequencies of the transmitted signal and the received signal substantially at 90°.
The duplexer includes a Bragg reflective-type duplexer and an air gap-type duplexer according to the FBAR. When a resonance part which generates a resonance is separated from a substrate part for better resonance efficiency of the FBAR, the Bragg reflective-type duplexer uses a reflection layer while the air gap-type duplexer uses an air gap for the separation of filters.
A Bragg reflective-type FBAR is formed in a manner that the reflection layer is formed by vapor-depositing materials of high acoustic impedance and low acoustic impedance alternately, and then, a lower electrode, a piezoelectric layer and an upper electrode are deposited on the reflection layer in order. The Bragg reflective-type duplexer is fabricated by integrating the Bragg reflective-type FBAR on a substrate. Therefore, the Bragg reflective-type duplexer can be implemented in a one-chip system and have a stable structure. However, the thickness of each layer is hard to minutely control, and the film easily gets cracked due to a stress caused by forming the thick reflection layer. Furthermore, the quality factor (Q) is considerably inferior compared to the air gap-type duplexer.
FIG. 1 is a plan view showing the structure of a conventional air gap-type duplexer adopting an air gap-type FBAR. Referring to FIG. 1, the air gap-type duplexer comprises an electrode terminal 10, a transmitter filter 20, an isolation part 30 and a receiver filter 40, all of which are integrated on the substrate. As described above, the transmitter filter 20 and the receiver filter 40 comprise the air gap-type FBAR, which is structured by depositing the lower electrode, the piezoelectric layer and the upper electrode in order on the air gap which is formed on the substrate. A plurality of the FBARs may be combined into one filter.
The isolation part 30, being separately fabricated, is integrated on the substrate part between the transmitter and receiver filters 20 and 40 in order to isolate signals passing through the filters 20 and 40. For this, the isolation part 30 operates as a phase shifter which makes the phase difference between the frequencies of the transmitted and received signals substantially at 90° by combining the inductor and the capacitor in a predetermined form, or forming a transmission line of λ/4 in length. Accordingly, a signal flow between the transmitter filter 20 and the receiver filter 40 can be prevented. The transmitter filter 20, the receiver filter 40 and the isolation part 30 are connected to the external electrode terminal through wire bonding.
According to the conventional duplexer, since the filters 20 and 40 and the isolation part 30 are separately fabricated and then integrated on a single substrate, complex manufacturing process was required. Further, the capacitor constituting the isolation part 30 is fabricated using a dielectric layer having a superior permittivity, however, since the manufacture of the dielectric layer includes a high-temperature requiring process, components may be damaged by heat.